Miniature motor for optical device based on electrowetting

ABSTRACT

Motor, comprising a first body ( 3 ) and second body ( 5 ), the latter being movably mounted with respect to the first body. A chamber ( 4 ), sandwiched between surfaces of said bodies, is filled with a non-polar and/or non-conductive first fluid ( 6 ) and at least one volume of a polar and/or conductive second fluid ( 7 ), which fluids are immiscible. One of said surfaces is provided with means ( 10 ) for locally varying the wettability of said surface by the second fluid, to move the or each volume of second fluid along a desired path. The other surface is provided with means ( 9, 14, 15 ) for coupling the or each volume of second fluid to this surface, so that this surface will be dragged along by the moving volume or volumes. This results in a relative movement between both surfaces and hence in a movement of the second body.

The invention relates to a motor, in particular a miniature motor, forrotation of for instance a mirror in an optical microswitch or afocusing or zoom system in for instance cameras or scanners.

Ongoing miniaturization of these and similar appliances has raised aneed for ever smaller motors. This need is presently met by downsizingexisting ‘normal size’ motors. However, as a rule of thumb the costs ofmanufacture rise as the size of the motor decreases, which rise is oftendisproportional. Moreover, some types of motors cannot easily beminiaturized or only up to a certain extent. For instance,miniaturization of electromotors based on coils and magnets is limitedto the point where the coils can no longer be wound. Other motors, forinstance those based on piezoelectric principles, can be miniaturizedbut are relatively expensive to manufacture.

It is an object of the invention to provide a motor, which can beminiaturized and manufactured in a cost effective way.

This object is achieved by the motor according to the invention, whichcomprises a first body, a second body movably mounted with respect tothe first body, a chamber situated between a surface of the first bodyand a surface of the second body, said chamber being filled with anon-polar and/or non-conductive first fluid and at least one volume of apolar and/or conductive second fluid, the fluids being immiscible,wherein one of said surfaces, to be called the first surface, isprovided with means for locally varying the wettability of said surfaceby the second fluid and the other surface, to be called the secondsurface, is provided with means for coupling the or each volume ofsecond fluid to the second surface.

The motor according to the invention makes advantageously use of knownwetting techniques for manipulating a volume of a fluid along apredetermined path. With these techniques, the surface tension of saidvolume is locally reduced, electrically, thermally or chemically,causing the volume to flow in the direction of its lowest surfacetension. This movement is subsequently conveyed to a movably mountedbody by coupling the volume of fluid to said body by suitable couplingmeans. In this way, the body will be dragged along by the moving volume.In more general wording: by using known wetting techniques to manipulatea volume, e.g. a droplet of fluid along a first surface and having thisvolume adhere to a second surface, one of these surfaces (belonging tothe movably mounted body) can be moved relative to the other surface(belonging to the static body).

In a first preferred embodiment according to the features of claim 2,the coupling between said volume of fluid and said second surface isachieved through wetting forces, induced by providing the second surfacewith at least one permanent or temporary area of high wettability bysaid fluid.

In a second preferred embodiment according to the features of claim 3,the coupling is achieved through surface tension forces. To that end,the second surface is provided with at least one recess, filled withsaid fluid. The volume may be coupled to this fluid through surfacetension forces, whereas the fluid may be anchored in the recess throughsuitable recess design.

A combination of the two coupling principles is possible as well. Forboth embodiments, the external forces to overcome to move the movablebody should not exceed the surface tension forces of the volume, becausethis would cause the volume to split up and remove the coupling.

A motor based on the above described principles offers the advantagethat it can be relatively easily miniaturized and manufactured costeffectively, thanks to the absence of complicated components. In fact,the most critical aspects of the motor will be the dimensions of thechamber between the first and second body and the positioning of themeans for varying the wettability, which determine the path of thedroplets and consequently the movement of the motor. The chamber ispreferably of capillary dimensions. For most fluids this meansdimensions of the order of several millimeters at most. With present daymanufacturing techniques such dimensions and any tolerances associatedtherewith can be easily accomplished. A motor according to the inventionfurthermore offers reliable, smooth and wear-free operation thanks tothe absence of dry friction. Also, in respect of the dimensions of themotor, relatively large displacements are possible.

According to a preferred embodiment, a motor according to the inventionis characterized by the features of claim 4.

The use of electrodes to vary the wettability of the first surface (andpossibly that of the second surface in case of the first preferredembodiment), offers the advantage that electrodes can be easily driven,in any desired sequence, with relatively low voltages. Power consumptioncan be low, resulting in an energy effective motor. Further, theelectrodes can be easily manufactured, at relatively low cost and inrelatively small sizes, for instance by known etching techniques. Also,the electrodes offer great freedom in possible motor movement, becausethe electrodes can be positioned in any desired pattern and activated inany desired order to force a volume of fluid along a desired path. Sincethe movement of the movable body will largely correspond to the path ofmovement of said volume of fluid, it will be clear that complex motormovements can be accomplished just by proper arrangement and activationof the electrodes. Furthermore, the electrodes keep the volume in place,as a consequence of which no fixed (physical) channels are needed. Thiscontributes to the simplicity of the motor configuration.

In a further preferred embodiment, a motor according to the presentinvention is characterized by the features of claim 7.

The use of a liquid metal as second fluid, for instance mercury, offersthe advantage that such fluids usually have a very high surface tension,which prevents the volumes from premature splitting up. This of courseis especially advantageous when the external forces on the body to bemoved are expected to be relatively large.

In an alternative preferred embodiment, a motor according to the presentinvention is characterized by the features of claim 10.

The use of an aqueous solution as a second fluid offers the advantagethat the wetting force of such solutions is very high on some materials(e.g. glass) and their surface tension is relatively high as well.Furthermore, aqueous solutions, thanks to their non-abrasive natureimpose little restrictions on the other materials to be used, are easyto handle and in general quite harmless, so that no demanding protectiveprovisions are needed with regard to leakage. Moreover, droplets of anaqueous solution can be displaced with relatively low voltages andrelatively low power consumption.

If the second fluid is an aqueous solution, the first surface ispreferably covered with a layer of hydrophobic material which can belocally changed to hydrophilic with suitably arranged electrodes. Thesecond surface is preferably covered with alternating layers ofhydrophilic and hydrophobic material, to form areas of high and lowwettability respectively. Such layers of hydrophobic and hydrophilicmaterial can be easily applied with known coating techniques, forinstance by means of lithography. This technique also offers thepossibility of applying a pattern of hydrophilic material onto anotherwise hydrophobic surface, so as to form paths or channels for thevolumes of second fluid.

It will be clear to the skilled person that the number of volumes ofsecond fluid, or the volume itself can be increased to increase theattainable wetting force. It will furthermore be clear that theafore-described motor principle can be used to construct both rotary andlinear motors, by a proper design of the first and second bodies as wellas an appropriate design of the movement paths for the or each volume ofsecond fluid. Also, it will be clear that either the movable body or thestatic body can be provided with the means for varying the wettability.However, due to the wiring needed, mounting the means on the static bodywill usually be most convenient. Of course it is also possible toprovide both bodies with means for varying the wettability. This willmake the surfaces functionally interchangeable and hence adaptable toany given situation. In that case, it is also possible to activate thewetting means on both surfaces simultaneously but at a different pace,which will result in a sort of artificial skid. This may for instance beused for actively controlled deceleration of the movable body.

The invention furthermore relates to an optical device, comprising themotor according to the invention, for instance for driving a reflectiveelement.

Further advantageous embodiments of a motor according to the presentinvention are set forth in the dependent claims.

To explain the invention, exemplary embodiments thereof are hereinafterdescribed with reference to the accompanying drawings, wherein:

FIGS. 1A,B show in transverse cross section a rotary motor according toa first embodiment of the present invention, in two successivepositions;

FIG. 2 shows schematically, in longitudinal cross section a possibleapplication of the rotary motor according to FIG. 1 in an opticalscanner; and

FIG. 3 shows schematically a linear motor according to a furtherembodiment of the invention.

In this description, identical or corresponding parts have identical orcorresponding reference numerals. All combinations of parts of theembodiments shown and described are explicitly understood to beincorporated in this description.

In this description, the term ‘wetting’ is understood to encompass alltechniques causing the surface tension of a volume, e.g. a droplet of aspecific fluid to be locally varied, so as to influence the wettingbehavior of said fluid with respect to a specific surface. When thisinfluencing is done electrically (as opposed to for instance thermallyor chemically) the term ‘electrowetting’ will be used. Moreparticularly, the term electrowetting is understood to at leastencompass the process whereby an electric potential is applied across aninterfacial layer between a droplet and an electrode, causing thewetting behavior of the droplet to alter, in particular to improve. Theterm ‘wettability of a surface by a certain fluid’ is understood to givean indication of the ease with which said fluid may wet said specificsurface, which may for instance depend on the nature of and/or theelectric potential across said surface. If a surface has a ‘highwettability by a specific fluid’, this indicates that a droplet of saidfluid in contact with said surface will have a rather expanded shape,with a relatively large contact area and a relatively small contactangle, usually less than about 90°, whereas ‘low wettability’ indicatesthat the droplet in contact with said surface will have a rathercontracted shape, with a relatively small contact area and a relativelylarge contact angle, usually exceeding about 90°. If the specific fluidis an aqueous solution, the term high wettability will be replaced byhydrophilic and the term low wettability will be replaced byhydrophobic.

FIG. 1A,B shows a first embodiment of a motor 1 according to the presentinvention, in particular a rotary motor, comprising a substantiallycylindrical first body 3 and a substantially cylindrical second body 5,which is concentrically positioned within the first body 3. The firstand second body 3, 5 enclose between their respective inner and outersurfaces a substantially cylindrical chamber 4, which is filled with anon-polar and/or non conductive first fluid 6, for instance air or anoil, and volumes 7 a-d of a polar and/or conductive second fluid 7, inthis example an aqueous solution, for instance (salted) water. Bothfluids 6, 7 are immiscible.

The first body 3 is provided with means for varying the wettability ofits inner surface, namely twelve electrodes 10 extending in axialdirection of the first body 3, spaced at substantially regular radialintervals along the circumference. The inner surface of the first body 3is covered with a layer 12 of electrically insulating, hydrophobicmaterial or more generally: a material having a wettability by thesecond fluid 7 which is lower than the wettability by the first fluid 6.Examples of such material are for instance Teflon-like materials likethe amorphous fluoropolymer AF1600 provided by Dupont or parylene or acombination thereof, in case where the first fluid 6 is an oil or airand the second fluid is (salted) water. Alternatively, the first body 3can be made of said hydrophobic material, and the electrodes 10 may beembedded in the first body 3, just below its inner surface, so that theyare covered by a thin layer 12 of said hydrophobic material. Theelectrodes 10 are connected to a voltage supply (not shown).

The second body 5 is of solid design but could be hollow, if so desired,and is mounted movably, in particular rotatably, in the first body 3 byone or more suitable bearings. The or each bearing could for instance bean oil bearing, configured by providing the first and/or second body 3,5 with an annular groove, in which upon rotation of the second body 5,pressure will build up, centering the second body 5 in the first body 3.

The second body 5 is provided at its outer surface with coupling meansin the form of four hydrophilic areas 14, said number corresponding tothe number of volumes 7 a-d. These areas 14 could for instance be madeof or covered by a material having a wettability by the second fluid 7that is higher than the wettability by the first fluid 6. In the presentexample, given the selected first and second fluids 6, 7, this materialcould for instance be glass. The areas 14 are separated from each otherin radial direction by areas 15, made of or covered by hydrophobicmaterial, which could be a selection from any one of the materialsmentioned before. Additionally or alternatively, the hydrophilic areas14 may be recessed to enhance the coupling force with the volumes.Furthermore, two or more of the volumes 7 a-d could be interconnectedvia at least one suitable conduit 9 in second body 5, as illustrated inbroken lines in FIGS. 1A,B. Such conduit 9 can be easily manufactured.The areas of high and low wettability 14, 15 may be omitted, but canalso be maintained, to increase the maximum force the motor may exert.

A motor as described above operates as follows. In FIG. 1A theelectrodes 10 marked with Roman numerals I (that is the upper, lower,left and right electrodes) are supplied with a voltage. Consequently,the hydrophobic layer 12 covering said electrodes I will become locallyhydrophilic. The four volumes 7 a-d will therefore contact the firstbody 3 at the four electrodes I. They furthermore contact the secondbody 5 at the coupling means, that is the hydrophilic areas 14 and theconduits 9. If, subsequently, the voltage supply is shifted to secondelectrodes II, situated next to the former electrodes I, the layer abovesaid second electrodes II will become hydrophilic, whereas the layerabove the first electrodes I will switch back to hydrophobic. This givesrise to electrowetting forces which draw the volumes 7 a-d towards thehydrophilic areas II as shown in FIG. 1B. During this movement thevolumes 7 a-d will move along the hydrophilic area 14 of the second body5 up to the edge of the hydrophobic area 15. Further movement along thesecond body 5 will be blocked by the combined action of the hydrophobicarea 15 and the first fluid 6, enabling the volumes 7 a-d to exert awetting force on the second body 5, which will cause the body 5 torotate. Hence by sequentially activating successive electrodes 10 I, IIwith a suitable voltage, the second body 5 can be rotated continuously.Preferably, the electrodes 10 are positioned relatively close to eachother or even overlap through a ‘tooth’ structure. Also, the radialdimensions of the electrodes 10 are preferably equal to or smaller thanthe radial dimensions of the volumes 7 a-d. Such positioning and/ordimensioning of the electrodes 10 will ensure that the volumes 7 a-d can‘sense’ a newly supplied voltage to a succeeding electrode 10 II.

In the given example the rotation is clockwise. It will be appreciatedthat this direction can be readily reversed by reversing the order inwhich the electrodes 10 I, II are activated. Obviously, the frequency ofrotation will depend on the activation frequency of successiveelectrodes 10 I, II. It is noted that although in the illustratedexample four volumes 7 a-d of conductive fluid are used, this numbercould be any number. The volumes 7 a-d may be line-shaped in axialdirection or consist of a series of axially spaced droplets. It isfurther noted that with the embodiment of FIG. 1, it is also possible tohave the first body 3 rotate instead of the second body 5, provided thefirst body 3 is rotatably mounted and the second body 5 is fixed. Inthat case, upon switching the voltage from the first I to the secondelectrodes II, the volumes 7 a-d would move towards this secondelectrode II (featuring the higher wettability) as far as the edge ofthe hydrophilic area 14. Subsequently, the second electrodes II due towetting forces would be drawn to the volumes 7 a-d, causing the firstbody 3 to rotate anti-clockwise. From this discussion it is alsoimmediately clear that for the operation of the motor I it is irrelevantwhether the electrodes 10 are positioned on the static body or themovable body. Therefore, although in practice the electrodes 10 willusually be placed on the static body to avoid wiring problems, thepresented embodiment should in no way be seen as limiting.

A motor as described above offers several advantages. For instance, themotor can be manufactured cost-effectively, since all layers 12, 14, 15can be applied by relatively simple, known coating techniques, such aslithography. Furthermore, all parts of the motor have a relativelysimple configuration and are therefore suitable for far-reachingminiaturization. Also, the volumes 7 a-d do not require fixed, that isphysically restricted, channels. A suitable layout of hydrophobic andhydrophilic layers will suffice to keep the volumes in place. This addsto the simplicity of manufacture, as such layout of hydrophobic andhydrophilic layers can be easily applied by known aforementioned coatingtechniques. Furthermore, the motor can be very easily adjusted toperform a great number of different motor movements, as will beexplained in further detail below.

The embodiment shown in FIG. 1 can be easily converted into a linearmotor 1′, by rotating the orientation of the electrodes 10 over 90°,that is from a radial towards an axial orientation as shown in FIG. 3,in which part of the first body 3 is left out, for clarity's sake.Instead of the separate series of axially orientated electrodes 10,ring-shaped electrodes could be applied, as indicated in broken linesfor the first electrode 10A of the series. Furthermore, the alternatingareas of high and low wettability 14, 15 have been converted into ringshaped areas, alternating in the axial direction of the second body 5.Volumes of second fluid 7 are in contact with a ring 14 of highwettability and an activated electrode I at the first body 3. Uponactivation of the next electrodes II, the volumes 7 will move in axialdirection along the inner surface of the first body 3, dragging thesecond body 5 along in the axial direction A, thanks to the blockingaction of the ring-shaped areas 15 of low wettability (or couplingforces provided by recesses or conduits, not shown). Stop mechanisms canbe provided to limit the maximum stroke of the second body 5. Thevolumes 7 of second fluid may be shaped as droplets, spaced at regularintervals along the ring 14, as illustrated. However, the volumes 7 canalso be ring-shaped, so as to cover the ring-shaped area 14, resultingin an evenly and symmetrically distributed wetting force along thecircumference of the second body 5. The number of volumes 7 andring-shaped areas 14 in axial direction can also be any desired number.

From the above-described embodiments it will be appreciated, that byrearranging the position of the electrodes 10 along the circumference ofthe first body 3 and accordingly adapting the coupling means on thesecond body 5, the motor 1,1′ can be simply adjusted to perform a widevariety of movements. For instance, the features of the motors 1, 1′according to FIGS. 1 and 3 can be combined so as to create a motorhaving a movable body 5 that can rotate and translate, eithersequentially or simultaneously, the latter resulting in a spiral-likemovement. Moreover, if the electrodes 10 on the first surface arearranged in a grid and the coupling means 9, 14 on the second surfaceare configured as spots, each accommodated to couple a volume 7 ofsecond fluid, it becomes possible to drive the movable body 5 in anydesired direction. Freedom of movement can even further be increased byproviding the second surface too with a grid of electrodes, similar tothe ones of the first surface, with which hydrophilic areas 14 can becreated according to ones' needs. Hence, a motor 1, 1′ according to theinvention offers great flexibility in attainable motor movements, with astandard set of simple components.

FIG. 2 shows one possible application of a motor 1 according to FIG. 1,in an optical scanner 20. In this embodiment the cylindrical first body3 is near its ends 24, 26 made of transparent material. The second body5 is made of transparent material as well, for instance glass, androtatably mounted in the first body 3, for instance by means of one ormore oil bearings as described previously. A mirror 22 is mounted on topof the second body 5, including an angle of about 45° to thelongitudinal axis thereof. A light beam, entering the second body 3through its lower transparent portion 26, will reach the mirror 22through the transparent second body 5, be deflected over 90° and exitthe first body 3 through its upper transparent portion 24. Rotation ofthe second body 5 will result in a rotating spot. Such a scanner can forinstance be used in a catheter to scan inside surfaces of blood vessels.

The invention is not in any way limited to the exemplary embodimentsshown in the description and the figures. Many variations thereof arepossible within the scope of the invention.

For instance, the first and second body do not need to be cylindrical.These bodies can have any shape, as long as they are each provided witha surface, which can cooperate with the surface of the other body so asto form a chamber, in which volumes of a second fluid can contact bothsurfaces. For instance, one of the bodies could have a cup-shapedsurface whereas the other body could have a ball-shaped surface so as toform a cup-and-ball joint. The semi-spherical chamber enclosed betweensaid bodies could be filled with oil and droplets of water, which couldbe driven according to above-described motor principle, to have one ofthe bodies rotate in any desirable direction. Also, the number and shapeof the volumes of second fluid is not limited to the ones shown in theembodiments. More or fewer volumes are feasible, having any desirableshape. Furthermore, the first and second fluid can be of a differentmaterial. The second fluid may for instance be a liquid metal, such asmercury, whereas the first fluid may be an electrolyte, immiscible withmercury. In that case, each volume may be positioned between a pair ofelectrodes, with which an electrical field can be applied over thevolume, extending in the direction of intended movement of the volume.This field will cause the volume to move towards one of the electrodes.This movement can be prolonged into a continuous movement, by making useof overlapping electrode pairs, activated sequentially. The movingvolume can be coupled to the movable body in the same way as describedabove for dragging this body along, that is through wetting forcesinduced by appropriate alternating areas of high and low wettabilityand/or by interconnection of these volumes via conduits in the secondbody.

These and many comparable variations are understood to fall within thescope of the invention as set out in the appended claims.

1. A motor, comprising a first body (3), a second body (5) movablymounted with respect to the first body (3), a chamber (4) situatedbetween a surface of the first body (3) and a surface of the second body(5), said chamber (4) being filled with a non-polar and/ornon-conductive first fluid (6) and at least one volume of a polar and/orconductive second fluid (7 a-d), the fluids (6, 7) being immiscible,wherein one of said surfaces, to be called the first surface, isprovided with means for locally varying the wettability of said surfaceby the second fluid (7) and the other surface, to be called the secondsurface, is provided with means for coupling the or each volume ofsecond fluid (7 a-d) to the second surface.
 2. A motor according toclaim 1, wherein the coupling means comprise at least one area (14) ofhigh wettability by the second fluid (7), said area being bounded by anarea (15) of low wettability by said second fluid (7), at least in adirection of relative movement of said first and second surface.
 3. Amotor according to claim 1, wherein the coupling means comprise at leastone recess (9), which opens into the chamber (4) and is filled with thesecond fluid (7), so that the at least one volume of second fluid (7a-d) in the chamber (4) will be coupled to the second fluid (7) in therecess (9) through surface tension forces.
 4. A motor according to claim1, wherein the means for locally varying the wettability of the firstsurface and/or the second surface, comprise a series of neighboringelectrodes (10), separated from the second fluid (7) by an interfaciallayer (12), and means for sequentially powering successive electrodes(10) so as to apply an electric potential across said interfacial layer(12), causing the condition thereof to switch between low and highwettability by the second fluid (7).
 5. A motor according to claim 4,wherein the electrodes (10) are spaced at substantially regularintervals along an intended path of movement of the or each volume (7a-d) of second fluid.
 6. A motor according to claim 1, wherein thesecond fluid (7) is a liquid.
 7. A motor according to claim 6, whereinthe second fluid (7) is a liquid metal.
 8. A motor according to claim 7,wherein the first fluid (6) is an electrolyte.
 9. A motor according toclaim 8, wherein the first fluid (6) forms the interfacial layer.
 10. Amotor according to claim 6, wherein the second fluid (7) is an aqueoussolution, for instance water, more particularly salted water.
 11. Amotor according to claim 10, wherein the interfacial layer (12) is adielectric layer having a low wettability by the second fluid (7).
 12. Amotor according to claim 11, wherein the dielectric layer (12) is madeof hydrophobic insulating material.
 13. A motor according to claim 12,wherein the hydrophobic insulating material is AF1600 and/or parylene.14. A motor according to claim 10, wherein the first fluid (6) is a gas,for instance air, or a liquid, for instance oil.
 15. A motor accordingto claim 10, wherein the second surface is covered by or made ofhydrophobic material and provided with at least one area (14) ofhydrophilic material, to form an area having a low wettability by thesecond fluid (7).
 16. A motor according to claim 1, wherein the firstand the second body (3, 5) are both substantially cylindrical, whereinone of the bodies (3; 5) is concentrically received within the otherbody (5; 3) and the chamber (4) is enclosed between the inner surface ofthe outer body and the outer surface of the inner body.
 17. A motoraccording to claim 1, wherein the motor is a rotary motor (1), whereinthe second body (5) is arranged for rotating movement with respect tothe first body (3).
 18. A motor according to claim 1, wherein the motoris a linear motor (1′), wherein the second body (5) is arranged fortranslating movement with respect to the first body (3).
 19. A motoraccording to claim 16, wherein the electrodes (10) are spaced at regularradial intervals along the circumference of one of the bodies (3, 5).20. A motor according to claim 16, wherein the inner body (3; 5) is thesecond, moveable body (5).
 21. A motor according to claim 1, wherein thefirst surface belongs to the first body (3) and the second surfacebelongs to the second, moveable body (5).
 22. A motor according to claim1, wherein the chamber (4) between the first and second body (3, 5) isof capillary dimensions.
 23. A motor according to claim 1, wherein thechamber (4) comprises channels for the second fluid (7), said channelsbeing formed by covering the second surface with or making the secondsurface of a material with low wettability by the second fluid (7) andproviding the surface with a channel-constituting pattern of materialwith high wettability by the second fluid.
 24. Optical device,comprising a reflective element and a motor according to claim 1, formoving said reflective element.